Heating of commercial greenhouses is accomplished with a variety of different systems, the choice of which is usually made by the grower based on the type of plants being grown. For a small “backyard” type greenhouse, among the simplest heating systems is the hot water unit heater. This device consists of a hot water coil with a propeller fan attached. Air distribution is normally through a “poly tube”–a thin clear plastic tube perforated with holes. The tube is normally hung from the ceiling, but can also be installed on the floor. Unit heaters are a practical method of greenhouse heating at supply water temperatures down to approximately 140F to 150F. Below this range, heat output is reduced by more than 50% and the cost of larger equipment becomes an obstacle